5. Prism wash systems
5.1 Prism coating
Deposit build-up on the prism surface disturbs the measurement. Look out for an
abnormally high concentration reading or an upward concentration (CONC) drift.
In most applications the prism keeps clean due to the self-cleaning eect. If coating occurs,
check the following:
• Sucient flow velocity, see Pipe mounting checklist (page 20).
• A temperature dierence between process fluid and sensor probe can cause coating.
This can happen with small flows if the thermal insulation is inadequate. In some cases
it also helps to insulate the clamp connector.
In case of a coating problem, the preferred solution is to try to increase the flow velocity, for
example, by installing a pipe portion with smaller diameter.
Consider installing a wash nozzle, if increasing the velocity or using a flow booster does not
provide a solution. See Prism wash (page 37).
5.2 Prism wash
Three alternative media can be used for prism wash:
• Steam
• Water
• High pressure water
The built-in relays of the indicating transmitter can be configured to control the prism wash
cycle, see Configuring relays (page 66).
In food industry applications, wash media must be culinary steam or safe
water. Shut-o valve and check valve must meet 3‑A sanitary standards.
CAUTION!
Culinary Steam: Steam produced using a system meeting criteria in the 3‑A Accepted
Practices for a Method of Producing Steam of a Culinary Quality, Number 609.
Safe Water: Water from a supply properly located, protected, and operated, and of safe,
sanitary quality. The water must meet the standards prescribed in the National Primary
Drinking Water Regulation of the Environmental Protection Agency (EPA) as referenced in
The Code of Federal Regulations (CFR), Title 40, Parts 141, 142, and 143.
5.2.1
Recommended wash pressures and times
The following tables show the recommended wash pressures and times.
Chapter 5 – Prism wash systems
37
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